The philosopher's stone is no more to be found in the organic than the inorganic world; and we shall seek as vainly to transform the lower animal types into the higher ones by any of our theories, as did the alchemists of old to change the baser metals into gold.' He also says: 'To me the fact that the embryonic form of the highest vertebrate recalls in its earlier stages the first representatives of its type in geological times and its lowest representatives at the present day, speaks only of an ideal relation, existing, not in the things themselves, but in the mind that made them. It is true that the naturalist is sometimes startled at these transient resemblances of the young among the higher animals in one type to the adult condition of the lower animals in the same type; but it is also true that he finds each one of the primary divisions of the animal kingdom bound to its own norm of development, which is absolutely distinct from that of all others; it is also true that, while he perceives correspondences between the early phases of the higher animals and the mature state of the lower ones he never sees any one of them diverge in the slightest degree from its own structural character-never sees the lower rise by a shade beyond the level which is permanent for the group to which it belongs-never sees the higher ones stop short of their final aim, either in the mode or the extent of their transformation.